docs: def group_by_day(self):
          """Return a dictionary of this collection's values grouped by each day of year.
  
          Key values are between 1-365.
          """
          data_by_day = OrderedDict()
          for d in xrange(1, 366):
              data_by_day[d] = []
          for v, dt in zip(self._values, self.datetimes):
              data_by_day[dt.doy].append(v)
          return data_by_day